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Irish Potato Famine Pathogen Takes Gun to Knife Fight

An international team of researchers – including Dr. Jean Beagle Ristaino, professor of plant pathology at North Carolina State University – has completed the genome sequence of one of the most destructive and rapidly evolving pathogens in the world.

The research shows that Phytophthora infestans, the fungus-like pathogen responsible for the Irish potato famine in the 1840s as well as “late blight” disease currently decimating tomatoes and potatoes across the United States, has a larger and more complex set of genes than previously sequenced cousin Phytophthora genomes. The P. infestans genome – two-and-a-half to four times bigger than its cousins – contains contrasting regions. Some regions are densely packed with genes that don’t seem to change much, while some regions are sparsely populated with genes that seem to change rapidly.

Late blight disease on plants in Chile. Photo courtesy of Dr. Jean Ristaino.

Knowing more about how the pathogen infects plants – including the ways it learns to evade resistance genes specifically employed to halt its progress – and the mechanisms behind its rapid evolution can help scientists devise new ways of protecting plants.

The research is published in the Sept. 10 online edition of Nature.

The genome sequence shows that in the metaphorical “arms race” of pathogens versus their hosts, P. infestans seems to have an upper hand in its interactions with potatoes and tomatoes due to its large number of so-called effector genes. These effector genes – which appear to be located in the large genome’s sparsely populated regions – change quickly and have remarkable success at breaking down a host plant’s defenses, Ristaino says, including plants that have been specifically bred to withstand the pathogen’s attacks.

“The expanded regions appear to give the effector genes the ‘plasticity’ they need to evolve rapidly,” Ristaino says. “That allows them to overcome plant defenses.”

P. infestans is a particularly destructive pathogen that can destroy a field of potatoes or tomatoes within days. There are a number of strains, or types, of P. infestans; Ristaino has spent much of her research career as a “spud sleuth” who has identified the specific strain of the pathogen that caused the Irish potato famine in the 1840s. Her lab is currently working on collecting, archiving and genotyping this year’s outbreaks of late blight disease. Epidemics first occurred in the Southeast in May, but also spread rapidly in the Northeast on home garden tomato plants.

– kulikowski –

Note: An abstract of the paper follows.

“Genome sequence and comparative analysis of the Irish potato famine pathogen Phytophthora infestans

Authors: Jean Beagle Ristaino, North Carolina State University and numerous researchers from a international consortium. Lead and senior authors are Drs. Brian Haas and Chad Nusbaum, Broad Institute of MIT and Harvard, and Dr. Sophien Kamoun, Sainsbury Laboratory.

Published: Sept. 10, 2009, online in Nature

Abstract: Phytophthora infestans is the most destructive pathogen of potato and a model organism for the oomycetes, a distinct lineage of fungus-like eukaryotes that are related to organisms such as brown algae and diatoms. Here we report the sequence of the P. infestans genome, which at ~240 Mb is the largest and most complex yet sequenced in the chromalveolates. Its expansion results from a proliferation of repetitive DNA accounting for ~74% of the genome. Comparison to two other Phytophthora genomes revealed rapid turnover and significant expansions of specific families of secreted disease effector proteins, including many genes that are induced during infection or are predicted to have activities that alter host physiology. These fast evolving effector genes are localized to highly dynamic and expanded regions of the P. infestans genome. This likely plays a key role in the rapid adaptability of the pathogen to host plants and underpins its evolutionary potential.